Electrophoretic deposition involves a process of electrophoresis which is the motion of charged particles through a liquid medium under the influence of an applied electrical field. In some electrodeposition processes, the article to be coated is placed in an electrodeposition bath and made either the anode or the cathode, and a voltage is then applied between the anode and the cathode to produce an electric current and an electrodeposited coating.
Electrodeposition is a standard process for painting cars as well as appliances, machinery and a host of other articles. See, for example, Handbook of Electropaintinq Technology, Electrochemical Publications Ltd., 1978.
The art teaches that the current in a working electrolytic bath passes from the anode to the cathode. From some locations on the anode, the current passes straight to the cathode. From other locations, the current starts out almost in the opposite direction and takes a long curved arc in reaching the cathode. This results in high and low current density areas generated on the cathode with concomitant differences in coating thickness. The distribution of current in a plating bath can be calculated using techniques known to those skilled in the art and the resulting differences in thickness of deposition in the high current density areas compensated for by moving the electrodes in the bath or by shielding the electrodes in some manner. As used herein "high current density area" means those areas on a cathode where distribution of current is high as may be theoretically determined by one skilled in the art. See, for example, Mohler, J. B., Metal Finishing, March 1985, pp. 51-54.
In many electrodeposition processes, the typical deposition pattern shows thicker coatings on the edges of substrates, i.e., the high current density areas, and thinner coating towards the center, i.e., the low current density areas. See, e.g., Mohler, supra. The art also teaches that, for example, to control excessive deposition at the edges (i.e., high current density areas), the high current density areas should be shielded from the anode. See, for example, U.S. Pat. No. 4,162,955, directed to the electrodeposition coating of a continuous sheet of metal as it travels in a horizontal pathway between a pair of electrically charged electrodes. The patent discloses that excessive buildup of coating material at the longitudinal marginal edges of a traveling metal sheet, compared to the thickness of the coating material intermediate to the marginal edges, may be controlled by suspending a plurality of edge guards between the marginal edges of the metal sheet and adjacent electrodes so that the edge guards at least partially cover the marginal edges. The use of the edge guards produces a more uniform coating laterally across the traveling metal sheet. The edge guards are composed of any suitable electrically nonconductive material, such as a plastic. It should be noted that buildup opposite the high current density areas has typically been reported for electrodeposition of materials that remain conductive after plating.
The art also teaches that electrodeposition of compositions that do not remain conductive after electrodeposition, in other words, which become insulating, typically produce a uniform coating. Such deposits have a high electrical resistance, and at a given voltage, deposition is self-terminated for all practical purposes when the resistance of the deposited material reduces the current flow to insignificantly low value. It has been reported that the resistance of the insulator serves to provide leveling in that the area of greatest electrical attraction is coated first and the area of greatest current flows moves to more remote and electrically shielded areas as the resistance builds up on the earlier coated areas. See, e.g., U.S. Pat. Nos. 3,230,162 and 3,761,371.
Electrodeposition of photosensitive coatings is also generally known. See, for example, U.S. Pat. Nos. 3,738,835; 3,954,587; 4,029,561; 4,035,273 and 4,035,274. Furthermore, electrodeposition of photosensitive polymer compositions is disclosed in U.S. Pat. Nos. 4,414,311 and 4,592,816.
U.S. Pat. No. 4,592,816, hereby incorporated by reference, discloses photosensitive polymer compositions capable of being electrophoretically deposited on a conductive surface to form photoresists. Photoresists are photosensitive films capable of transferring an image onto a conductive substrate such as the metal surface of printed circuit boards. Such photoresists are useful in the preparation of printed circuit boards, lithographic printing plates, cathode ray tubes, as well as in chemical milling, solder resist, and planarizing layer applications.
In the fabrication of electrical components using photoresists, a planar or level photoresist coating is critical. Control of deposit thickness is important because the resulting thickness directly affects photospeed, development time, and yield per square foot. Thus, improved apparatus and methods for uniformly electrodepositing photoresist compositions are being sought.